A trunnion-type ball valve is known as a valve which is particularly suitable for a high-pressure fluid, and the ball valve is provided to have a structure in which: normally, a ball seat serving as a valve seat is attached to a body in a state in which the ball seat is retained by a seat retainer, the ball seat is subjected to elasticity in a valve-element direction by the elastic force of a coil spring, and, when the ball seat is pushed, a fluid is sealed by the ball seat in a primary side (upstream side).
In this manner, if the ball seat and the seat retainer are separated from each other, they are provided to have a structure in which fall-off of the ball seat from the seat retainer is prevented. For example, in a ball valve of Patent Literature 1, a seat is provided with a female thread, a seal ring is provided with a male thread, and the seal ring is attached to the seat via the female thread and the male thread. A disc spring is attached to a seat back surface, and the seat and the seal ring are pressed in a valve-element direction by the disc spring. As a result, in a state in which protrusion from the seat due to loosening is prevented by mutually contacting thread ridge ends thereof, the seal ring is brought into pressure-contact with a valve element by a compression coil spring attached to the back surface side, and the gap between the contact surfaces of the seal ring and the valve element is prevented.
Furthermore, in this ball valve, a seal member is provided in the inner diameter side of the seal ring, entry of the pressure into the back surface side is prevented by this seal member, and pressure entry to the seal-ring back surface caused by protrusion of the seal ring due to a back-pressure action is prevented.
In a ball valve of Patent Literature 2, a saw-teeth-like dent-bump part is formed on a packing liner, and, when a seat packing enters this saw-teeth-like dent-bump part, the seat packing is coupled to the packing liner. In this state, the seat packing is fixed in a state in which a back surface thereof is press-fitted with the packing liner, and, as a result, protrusion due to the back-pressure action of a fluid pressure is prevented. Furthermore, back leakage from the seat packing is also prevented by press-fitting of the seat packing with the packing liner.
In a ball valve of Patent Literature 3, a valve-seat retaining ring is engaged with the back-surface-side outer periphery of a valve seat, and a fluid communication groove is circumferentially provided on an engagement surface of the valve-seat retaining ring. The valve seat is configured to be pulled back to a normal position by equalizing the pressure difference between an inlet-side fluid-channel-part pressure and a pocket-part pressure via a fluid communication hole through this fluid communication groove. Thus, protrusion, deformation, and breakage of the valve seat are to be prevented.
On the other hand, a ball valve 1 shown in FIG. 14 is provided in a state in which fall-off of a ball seat 3 from a seat retainer 2 is prevented and is structured so that the ball seat 3 is attached to the seat retainer 2 in a free state.
Also, a ball valve of this type is sometimes required to be openable/closable while a ball seat and a valve element maintain a high sealing characteristic in addition to prevention of fall-off of the ball seat. For example, in the ball valve of Patent Literature 1, in order to ensure a sealing characteristic in addition to above described prevention of protrusion of the seal ring, a seal surface of the ball seat is provided to have the same curvature as that of a seal surface of the valve element, and the seal surfaces of the ball seat and the valve element having the same curvature are provided so as to cohere to each other across the entire peripheries thereof.
Also in the ball valve of FIG. 14, as well as Patent Literature 1, a seal surface 6 formed on the ball seat 3 in a range from a primary-side seal part 4 to a secondary-side seal part 5 is provided to have the same curvature as that of a valve-element seal surface 8 at the outer periphery of a ball valve element 7.
More specifically, in the ball valve thereof, in FIG. 14, fall-off of the ball seat 3 is prevented, and the spherical shape of the ball valve element 7 and the shape of the seal surface 6 of the ball seat 3 are provided to have the same dimension; therefore, the seal surface 6 is always in a state of surface contact with the valve-element seal surface 8 in the range of the primary-side seal part 4 to the secondary-side seal part 5 in a case of low pressure to a case of high pressure so that a fluid pressure is sealed.
Patent Literature 4 discloses a ball valve which is to prevent deviated abrasion of a ball seat and prevent reduction in a sealing characteristic by retaining the ball seat by a seat retaining part in a rotatable state in a circumferential direction and rotating the ball seat in the circumferential direction along with the rotation of a ball.
In a ball valve of Patent Literature 5, the space between a bottom surface of a seat ring and a bottom surface of a seat-ring retaining groove of a seat retaining means is provided so as to be widened and, at the same time, tilted to a ball side as it gets close to the outer diameter, and pressing deformation of a case in which pressing force is applied to the seat ring is absorbed by the space so as to prevent the position of a seal part from being changed and to retain a sealing characteristic.
In each of the above described trunnion-type ball valves, generally, a resin material is used as the ball seat, and, in this case, if the temperature becomes high, the fluid is sealed while the ball seat is elastically deformed by the fluid pressure depending on the pressure thereof.